The invention relates generally to wind turbines, and particularly to systems and methods for mounting a wind turbine superstructure to a pile foundation or a substructure at an offshore location. Specific embodiments of the present technique provide configurations, designs, and methods for driving a flanged pile (or caisson) suitable for supporting a wind turbine at an offshore location.
Wind turbines are generally regarded as an environmentally safe and desirable source of renewable energy. In summary, a wind turbine harnesses the kinetic energy of wind and transforms this kinetic energy into electrical energy. Thus, electrical power can be generated with virtually zero emissions, unlike existing natural gas-fired or coal-fired power generation technologies. To maximize the efficacy of power generation and to simplify connection to a power grid, several wind turbines are often located in proximity to one another in what are generally referred to in the pertinent art as “wind farms.” Advantageously, these wind farms are located in regions having relatively strong winds, such as, for example, at offshore locations.
An important criterion for establishing competitive offshore wind farms includes development of cost efficient foundations for the wind turbines. Typically, the foundation structure for a wind turbine at an offshore location comprises a monopile. A traditional monopile generally has a long cylindrical caisson that is assembled in sections on-shore and subsequently driven to the required penetration depth at the turbine's offshore location. Using the monopile as a foundation structure is advantageous because, for one reason, the same heavy lift equipment can be used for both the driving of the large monopile, and the installation of the wind turbines, thus facilitating an assembly line type of operation involving relatively few and standardized operations. Oftentimes these heavy lift equipment are procured as rentals by the production company, thus minimizing the usage time of these heavy lift equipment can reduce overall costs for the production company.
In certain offshore installations, the monopile is provided with a transition piece comprising a flanged structure attached to the top of the monopile. The transition piece provides a mating surface to which a wind turbine tower may be bolted. The transition piece may also be adapted to compensate for excess inclination in the driven monopile (an inclination in the superstructure of less than 0.5 degrees is generally desirable). For an offshore installation of a wind turbine to be cost efficient, it is desirable that the wind turbine tower is mounted on the monopile immediately after the monopile is driven. Unfortunately, past methods for mounting the transition piece on to the monopile which include grouting, swaging, or in-situ welding, generally require a significant amount of time, whether due to curing or installation, etc. That is, all of these traditional processes are relatively expensive and time consuming, leading to considerable idling of rented heavy lift equipment. Moreover, grouting involves a large surface overlap between the transition piece and the caisson (sometimes as high as 12-15 meters of overlap), leading to considerable expenditure on steel due to increased size of the transition piece.
A possible solution to the above problem could be to drive the monopile with a flanged structure welded at the top. However, at the time of driving the monopile by a hydraulic hammer, large accelerations and stresses are induced in the monopile, which may cause large deformations on the flange surface. These deformations may be so large that they are beyond the accepted tolerances for flange surfaces. With these accelerations, fatigue damage may also accumulate that may adversely impact the life of the foundation system.
Accordingly, there exists a need for a cost effective method for installing a foundation structure for a wind turbine at an offshore location.